The present invention resides in the removal of chlorate from caustic solutions with iron and the consecutive recovery of said iron in metal form.
The removal of unwanted excess chlorate in sodium hydroxide solutions obtained from the diaphragm electrolysis of brine is carried out by using reducing agents, a number of which have been described, including iron. Thus, U.S. Pat. No. 2,404,453 discloses the reduction of chlorate in sodium hydroxide solutions containing about 50 percent NaOH using iron in comminuted form (chips or turnings), said iron particles being coupled with a more noble metal such as copper. Also, U.S. Pat. No. 2,403,789 discloses the purifying of 30-60 percent NaOH solution from excess chlorate by means of iron powder or filings. In such processes, the iron dissolves in the caustic forming Fe.sup.+2 and Fe.sup.3+ ions and simultaneously reducing the chlorate to chloride.
The dissolved iron which is then present as an impurity in the caustic solution as the result of the removal of chlorate should thereafter be eliminated by means involving, for example, oxidation, precipitation or electrolytic reduction. Thus, the cathodic removal of iron and other metal ions has been disclosed in the following publications: U.S. Pat. No. 3,244,605 and French Pat. No. 1,505,466. According to the prior art, the caustic solution containing the iron to be removed (and possibly other metal impurities) is subjected to electrolysis whereby the iron deposits as a metallic coating on the cathode of the electrolytic cell. However, it has not been reported that such electrolytically deposited iron can be re-used for again reducing the chlorate content of a yet untreated caustic solution.